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From: Nadav Amit <namit@vmware.com>
To: Ingo Molnar <mingo@redhat.com>
Cc: <linux-kernel@vger.kernel.org>, <x86@kernel.org>,
	Nadav Amit <namit@vmware.com>,
	Christopher Li <sparse@chrisli.org>,
	<linux-sparse@vger.kernel.org>
Subject: [PATCH v8 03/10] x86: objtool: use asm macro for better compiler decisions
Date: Tue, 18 Sep 2018 14:28:40 -0700	[thread overview]
Message-ID: <20180918212847.199085-4-namit@vmware.com> (raw)
In-Reply-To: <20180918212847.199085-1-namit@vmware.com>

GCC considers the number of statements in inlined assembly blocks,
according to new-lines and semicolons, as an indication to the cost of
the block in time and space. This data is distorted by the kernel code,
which puts information in alternative sections. As a result, the
compiler may perform incorrect inlining and branch optimizations.

In the case of objtool, this distortion is extreme, since anyhow the
annotations of objtool are discarded during linkage.

The solution is to set an assembly macro and call it from the inline
assembly block. As a result GCC considers the inline assembly block as
a single instruction.

This patch slightly increases the kernel size.

   text	   data	    bss	    dec	    hex	filename
18140829 10224724 2957312 31322865 1ddf2f1 ./vmlinux before
18140970 10225412 2957312 31323694 1ddf62e ./vmlinux after (+829)

Static text symbols:
Before:	40321
After:	40302	(-19)

Cc: Christopher Li <sparse@chrisli.org>
Cc: linux-sparse@vger.kernel.org
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/kernel/macros.S |  2 ++
 include/linux/compiler.h | 56 ++++++++++++++++++++++++++++++----------
 2 files changed, 45 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/macros.S b/arch/x86/kernel/macros.S
index cfc1c7d1a6eb..cee28c3246dc 100644
--- a/arch/x86/kernel/macros.S
+++ b/arch/x86/kernel/macros.S
@@ -5,3 +5,5 @@
  * commonly used. The macros are precompiled into assmebly file which is later
  * assembled together with each compiled file.
  */
+
+#include <linux/compiler.h>
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 681d866efb1e..1921545c6351 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -99,22 +99,13 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
  * unique, to convince GCC not to merge duplicate inline asm statements.
  */
 #define annotate_reachable() ({						\
-	asm volatile("%c0:\n\t"						\
-		     ".pushsection .discard.reachable\n\t"		\
-		     ".long %c0b - .\n\t"				\
-		     ".popsection\n\t" : : "i" (__COUNTER__));		\
+	asm volatile("ANNOTATE_REACHABLE counter=%c0"			\
+		     : : "i" (__COUNTER__));				\
 })
 #define annotate_unreachable() ({					\
-	asm volatile("%c0:\n\t"						\
-		     ".pushsection .discard.unreachable\n\t"		\
-		     ".long %c0b - .\n\t"				\
-		     ".popsection\n\t" : : "i" (__COUNTER__));		\
+	asm volatile("ANNOTATE_UNREACHABLE counter=%c0"			\
+		     : : "i" (__COUNTER__));				\
 })
-#define ASM_UNREACHABLE							\
-	"999:\n\t"							\
-	".pushsection .discard.unreachable\n\t"				\
-	".long 999b - .\n\t"						\
-	".popsection\n\t"
 #else
 #define annotate_reachable()
 #define annotate_unreachable()
@@ -299,6 +290,45 @@ static inline void *offset_to_ptr(const int *off)
 	return (void *)((unsigned long)off + *off);
 }
 
+#else /* __ASSEMBLY__ */
+
+#ifdef __KERNEL__
+#ifndef LINKER_SCRIPT
+
+#ifdef CONFIG_STACK_VALIDATION
+.macro ANNOTATE_UNREACHABLE counter:req
+\counter:
+	.pushsection .discard.unreachable
+	.long \counter\()b -.
+	.popsection
+.endm
+
+.macro ANNOTATE_REACHABLE counter:req
+\counter:
+	.pushsection .discard.reachable
+	.long \counter\()b -.
+	.popsection
+.endm
+
+.macro ASM_UNREACHABLE
+999:
+	.pushsection .discard.unreachable
+	.long 999b - .
+	.popsection
+.endm
+#else /* CONFIG_STACK_VALIDATION */
+.macro ANNOTATE_UNREACHABLE counter:req
+.endm
+
+.macro ANNOTATE_REACHABLE counter:req
+.endm
+
+.macro ASM_UNREACHABLE
+.endm
+#endif /* CONFIG_STACK_VALIDATION */
+
+#endif /* LINKER_SCRIPT */
+#endif /* __KERNEL__ */
 #endif /* __ASSEMBLY__ */
 
 #ifndef __optimize
-- 
2.17.1


  parent reply	other threads:[~2018-09-18 21:30 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-18 21:28 [PATCH v8 00/10] x86: macrofying inline asm for better compilation Nadav Amit
2018-09-18 21:28 ` [PATCH v8 01/10] xtensa: defining LINKER_SCRIPT for the linker script Nadav Amit
2018-09-18 21:28 ` [PATCH v8 02/10] Makefile: Prepare for using macros for inline asm Nadav Amit
2018-09-26  8:58   ` Rasmus Villemoes
2018-09-26 17:56     ` Nadav Amit
2018-09-27  7:52       ` Rasmus Villemoes
2018-10-01 22:01       ` Masahiro Yamada
2018-10-03 19:41         ` Nadav Amit
2018-10-02 10:59       ` Ingo Molnar
2018-10-03 19:43         ` hpa
2018-10-03 20:29           ` Nadav Amit
2018-09-18 21:28 ` Nadav Amit [this message]
2018-09-18 21:28 ` [PATCH v8 04/10] x86: refcount: prevent gcc distortions Nadav Amit
2018-09-18 21:28 ` [PATCH v8 05/10] x86: alternatives: macrofy locks for better inlining Nadav Amit
2018-09-18 21:28 ` [PATCH v8 06/10] x86: bug: prevent gcc distortions Nadav Amit
2018-09-18 21:28 ` [PATCH v8 07/10] x86: prevent inline distortion by paravirt ops Nadav Amit
2018-09-18 21:28 ` [PATCH v8 08/10] x86: extable: use macros instead of inline assembly Nadav Amit
2018-09-18 21:28 ` [PATCH v8 09/10] x86: cpufeature: " Nadav Amit
2018-09-18 21:28 ` [PATCH v8 10/10] x86: jump-labels: " Nadav Amit
2018-09-21 18:26 ` [PATCH v8 00/10] x86: macrofying inline asm for better compilation Kees Cook

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